Maybe they just changed it? You posted this only 2 hours ago, but "Give me a knife" was accepted for me.
Because there's no ek chaakoo so it cant translate to a knife. In Hindi, the objects in those sentences are always particular as illustrated in Engish with the help of the unless they don't have a ek in the sentences.
Here's what I did to answer those exact questions: I opened a new browser window. I happen to have https://www.google.com as my starting page. I think that's default for Chrome. Then I typed in "chaku hindi". From the response I was able to determine the gender of चाकू with a high level of confidence. I was still uncertain about the plural form so I went to https://translate.google.com to translate "three knives" from English to Hindi. As a result I'm fairly confident about the plural form of चाकू as well. The big question now is whether I will be able to retain this information for any substantial length of time. If not I can always post a question on Duolingo.
Lol! It seems that you are indirectly hinting me to check such things in the net. Anyway okay. Duolingo forums are created for such doubts.
चाकू is of the masculine gender and the plural form remains the same - चाकू, which makes me think it is most likely a loan word.
BTW, to figure out gender, I will sometimes use google translate and add an adjective to the word to get a clue, such as "small knife", "small knives". It comes in pretty handy.